1. Field of the Invention
This invention relates to a rubbing cloth for use in manufacturing liquid crystal display panels. In the manufacturing process of liquid crystal display panels, an alignment treatment process is conducted by using a rubbing roller. The rubbing roller consists of a metal roller and a rubbing cloth fastened on peripheral surface of the meta roller with a double-sided adhesive tape The rubbing cloth has a pile of raised fibers. The alignment treatment process is a process for providing unidirectional molecular orientation on the surface of alignment layer by uniformly rubbing the surface of the alignment layer with the pile on the above rubbing roller rotating at a high speed wherein the alignment layer was formed on each of two substrates which is constituents of a liquid crystal display panel. This process is called alignment process or rubbing process. This invention relates to a rubbing cloth used in the rubbing process which is indispensable to manufacturing of liquid crystal display panels.
The rubbing process is explained with referring to FIG. 5 as follows. The rubbing process is an operation of rubbing the surface of an alignment layer formed on substrate 5, which is a constituent of a liquid crystal display panel, with a pile of cloth 2 on surface of metal roller 1 rotating at a high speed. The cloth 2 has been fastened on peripheral surface of metal roller 1 with a double-sided adhesive tape, and the cloth 2 has a pile of raised fibers. This rubbing process is conducted for providing unidirectional molecular orientation on the surface of alignment layer. The mark “d” in FIG. 5 denotes a pushing depth of cloth 2 onto surface of substrate 5, which is commonly called a depth of bite. The level of depth of bite d is most influential on alignment performance.
2. Description of the Prior Art
Transmission-type liquid crystal display panels are composed of a TFT substrate, CF substrate and liquid crystal which is encapsulated in the small gap between these two substrates. The TFT substrate has pixel driving devices made of thin-film transistor (TFT) array. The CF substrate has an area patterned color filter (CF) layer. On the TFT substrate, patterned ITO film is fabricated as pixel electrodes which are entirely covered with an alignment layer.
The TFT substrate and the CF substrate are assembled in face-to-face fashion where the two alignment layers directly sandwitch the encapsulated liquid crystal. The alignment layers on surfaces of TFT substrate and CF substrate have been subjected to an aligning treatment process by using a rubbing cloth in order to bring liquid crystal molecules into alignment. Usually, the rubbing cloth is fastened on peripheral surface of aluminum or stainless-steel roller with a double-sided adhesive tape. The rubbing cloth on the peripheral surface of the roller, while the roller is rotated, is brought into contact with the alignment layer surface to rub the alignment layer surface with the rubbing cloth. By conducting such rubbing treatment for the alignment layers, ultrafine grooves are formed on the alignment layer surface, or unidirectional molecular orientation is appeared in polyimide molecules which are macromolecules for forming the alignment layer. When the liquid crystal is brought into contact with the rubbing-treated alignment layers, the liquid crystal molecules can be aligned in the direction in which the alignment layer has been rubbed with the rubbing cloth. Accordingly, uniform switching characteristics induced by electric field of the liquid crystal display panel has been achieved. The performance of liquid crystal alignment provided by the rubbing treatment is influential on uniformity of display characteristic which is the most important characteristic for liquid crystal display panels.
As the rubbing cloth, a velvet fabric is commonly used which comprises a ground sheet and a pile of raised fibers woven to the ground sheet. As materials of the pile, long fibers (filaments) such as rayon and nylon and short fibers such as cotton are known in the art. As texture of velvet used for the rubbing cloth, a velvet fabric comprising a fast pile shown in FIG. 7 is commonly used (refer to Japanese Patent No. 3209328). In the velvet fabric, cupra rayon is used as the warps and wefts of ground sheet texture, and viscose rayon is used as the pile. Furthermore, a velvet fabric comprising a V-shaped pile (cut pile) shown in FIG. 8 is also used (refer to Japanese Registered Utility Model No. 3045464). In the velvet fabric, cotton is used as the pile. However, the texture of velvet is not limited to these combinations. For example, it is possible to weave a fast pile texture even if cotton is used as the pile. In prior arts, cellulose fibers have been used as ground yarns of velvet fabric which is used as a rubbing cloth. There is no prior art in which hydrophobic synthetic fibers or thermoplastic or hot-melt synthetic fibers are used as ground yarns.
Regarding to rubbing cloths consisting of the above types of velvet fabric, an effective method is known for preventing unevenness in the alignment due to uneven rubbing, which is a method wherein the pile is inclined to such a direction that the tip of pile is inclined from the position of root of pile (refer to Japanese Registered Utility Model No. 3032820). However, in the above described velvet fabric, the pile cannot be inclined at a desired angle only by effect of the woven texture.
As a method for inclining a pile of rubbing cloth in prior arts, for example, Japanese Registered Utility Model No. 3032820 discloses such method in detail for a velvet rubbing cloth wherein rayon is used as the pile. Specifically, it discloses a method comprising steps of immersing a velvet fabric obtained by ordinary weaving process in a cellulose-reactive glyoxal resin, combing the velvet fabric with a brush in a curing machine, and then heating and curing the resin to fix the inclination of pile. This method is called resin impregnation. Furthermore, this method is also effective for applying to regenerated cellulose fibers and fibers consisting of cellulose derivatives (i.e., cellulose fibers) besides rayon, such as cotton, polynosic fibers and lyocell because the cellulose-reactive glyoxal resin is used in the method. However, there are some problems in the method for inclining a pile by resin treatment, and the problems have to be solved in view of high definition and high yield in manufacturing liquid crystal display panels.
Furthermore, regarding to rubbing cloths comprising a cotton pile, there is no effective method for uniformly inclining the pile, except the same method as the above resin impregnation to rubbing cloths comprising a rayon pile.
In the above described resin treatment of the prior art, there is a high possibility that the treated shape and characteristics of the cloth are different between lots. Thus, in the manufacturing process of liquid crystal display panels, a new arrival of velvet fabric is previously put in an experimental use for rubbing in order to check the alignment quality and confirm whether the velvet fabrics in the lot can be used or not. After the check, the velvet fabrics in the lot are used in the manufacturing process. However, even if such check was done, unexpected rubbing faults occurs because unevenness of quality in the cloths themselves are not improved.
Furthermore, in conventional velvet of rayon, there is another problem that is concerning the resin itself. Specifically, since rubbing cloths (velvet) are immersed in a cellulose reactive resin, the resin is adhered to the surface of each of the short fibers that constitute the pile of velvet. This resin tends to be relatively hard and brittle. Consequently, it tends to wear away and fall during the rubbing treatment and adhere on the alignment layer surface as a contaminant, and it results in the occurrence of defects such as faulty display (refer to H. Tabira and T. Inoue et al. “Precision Rubbing Supported by Fine Process Analysis” Journal of the SID Vol.10, No.4, 329-337(2002)). Furthermore, the contaminants caught in the pile sometimes cause scratch defects on the alignment layer surface.
Furthermore, since the cellulose reactive resin is an aqueous emulsion, the rubbing cloths using cupra as ground yarns are easy to deform when they are in the moist state. This may cause a bow-shaped deformation (commonly known as bowing) in the wefts of the rubbing cloths, which is in turn easy to cause what is call a texture curving where the wefts do not intersect the warps at right angles.
These are the reasons why there are strong demands for means that can provide a pile with a stable inclination at high accuracy while avoiding the use of the resin impregnation.
As a method of forcing the pile to incline while avoiding the resin impregnation treatment, a method is known, in which, after forming a pile of thermoplastic synthetic fibers, the pile is combed with a brush in the intended direction while the back side of the velvet fabric is heated by contacting a heating cylinder, and then cooled so that the pile is fixed. However, since this method is easily affected by the temperature and humidity of open air, it is difficult to mass produce rubbing cloths stably with the inclination angle of their pile kept constant. Furthermore, if this method is applied to velvet fabric with a pile of cellulose fibers such as cotton, it is difficult to provide the pile with a stable inclination angle because the cellulose fibers are not thermoplastic.
As described above, in conventional methods of providing a pile with an inclination, their application is limited to certain pile materials, or even if they are applicable, there still remain unsolved problems inherent in the methods themselves.
In a velvet with a fast pile as shown in FIG. 7, when it is tried to incline the pile in the direction parallel to the warps, the pile tends to be inclined in the direction which deviates a little from the direction parallel to the warps (not parallel to the warps). In this case, it is known that if the rubbing cloth is fastened on a rubbing roller without allowing for the above described deviation, in such a manner as to make the warps direction (weaving direction) correspond to the direction in which the rubbing roller is rotated, the pile contacts small irregularities on the object of rubbing treatment and vibrates to cause alignment specks in rows in the alignment layer.
Thus, in conventional methods, when cutting off rubbing cloths from raw woven fabric, it is necessary to cut them at a little angle from the warp direction (weaving direction) with allowing for the above described deviation. However, it causes a problem of increased loss of fabric.
The cause of the deviation of pile's inclination from the direction parallel to the warps, which occurs in velvet with a fast pile, is described in connection with FIG. 9. FIG. 9 is an enlarged view of the circled portion of the velvet of FIG. 7 and corresponds to the cross section seen from the side parallel to the wefts. With regard to the relation between one weft 30 and two adjacent warps 31 and 32, on the woven texture, one warp 32 of the two adjacent warps is positioned on the front side of the weft 30 while another warp 31 is positioned on the back side of the weft 30 as shown in FIG. 9. Thus, the weft 30 passing between the two warps is not kept level, but inclined a little (slants to the left in FIG. 9). As a result, the pile intended to be raised perpendicular to the wefts is inevitably inclined. This leads to the deviation of the direction in which the pile is inclined from the warp direction.
When a natural fiber such as cotton is used for one or both of the ground warps and ground wefts of a rubbing cloth, it cannot be usually done to completely remove cotton waste such as withered stalk, leaf and petal in the manufacturing process because the fiber used therein is a natural fiber. As the result, the natural fiber containing cotton waste is woven as it is. If a rubbing cloth wherein cotton waste remains in the ground sheet is used for rubbing treatment process, the surface of alignment layer is injured by the cotton waste which fell on the surface or by change of total thickness. As the result, the uniformity of display characteristic is decreased, which is the most important characteristic for liquid crystal display panels. When a short fiber such as cotton is used for ground yarns, the weaving processability is decreased because it is apt to appear nap compared with a long fiber. As the result, there is a limitation of diameter of fiber used for ground yarns, and it is difficult to increase the density of pile in a warp direction of the specified woven texture. Furthermore, the elongation by stress is large. Especially, in manufacturing process of liquid crystal display panels under moist state, the elongation is remarkably large.
There is also some problems regarding to dimensional instability when a natural fiber is used for a rubbing cloth. Concretely, the problems are concerning that the sheet elongates through rubbing treatment process and that the dimension of sheet is changed after cutting process.
In the liquid crystal display devices which were formed of substrates whose alignment layers were rubbing-treated with a rubbing cloth of velvet fabric, non-uniform brightness may sometimes occur in the direction parallel to the rotation axis of rubbing roller. It is reported that one of the causes of the occurrence of non-uniformity is the elongation of velvet and slackening of sheet (refer to Japanese Patent No. 3209328).
In the manufacturing process of liquid crystal display panels, there are many cases that the relative humidity in the atmosphere is set rather high (about 60% or more) to decrease the generation of static electricity in order to prevent static damage to the TFT elements. On the other hand, in rubbing cloth used at present, there are many cases that cupra rayon is used as the fibers of ground cloths. The cellulose fibers such as cupra rayon and viscose rayon are easy to elongate in a high humidity condition rather than hydrophobic synthetic fibers (refer to “Textile Basic Engineering [I]”, p55, published by The Textile Machinery Society of Japan on Jul. 30, 1969). The elongation is a problem which is difficult to be solved.
In a cutting process of the rubbing cloth, it is very important to exactly cut it at a desired angle to the warps direction. In the prior arts, the cutting process is conducted on the assumption that the wefts intersect the warps at right angles. However, a texture curving frequently occurs in a finishing processes such as desizing and refining processes and resin-coating processes to the back side of the cloth. The texture curving particularly occurs near the both selvedges of cloth. This seems to be one of causes that the alignment properties of the rubbing treated liquid crystals are different between lots of the rubbing cloths.
Furthermore, after the rubbing cloth was cut, it is very important that the length of the rubbing cloth is not changed in its longitudinal direction because the rubbing cloth is fastened on peripheral surface of rubbing roller with a double-sided adhesive tape. Concretely, after the rubbing cloth was wound around the rubbing roller, the shrinkage of the rubbing cloth causes an opening on the joining part, or the elongation causes an overlap on the joining part or a slack of the cloth. These cause partial rubbing faults.
For manufacturing a rubbing roller, a raw woven fabric is made by using a weaving machine at a predetermined weaving width, and the raw woven fabric is cut into a desired size which fits with the size of a roller, and then it is fastened on surface of the roller with a double-sided adhesive tape. As a method of cutting the raw woven fabric in the above processes, a press cutting is conventionally done at ambient temperature. When the press cutting is done, there is a case that pile near the cutting portion comes off and falls on the cloth even if acrylic resin or polyvinyl acetate resin was coated on the back of the rubbing cloth wherein the back has no pile. Furthermore, even if the pile which came off and fell was removed, there is a case that the pile further comes off and falls during the rubbing process when the warps or wefts on the edge have frayed. These fibers which fell or frayed, contact to the alignment layer surface in the rubbing process. As the result, the alignment layer is injured, and the uniformity of display characteristic may be decreased, which is the most important characteristic for liquid crystal display panels.
Especially, the problems frequently occur when the rubbing cloth is cut in the direction not parallel to the textile direction (the warp direction). Such cutting in the direction not parallel may be necessary when the rubbing cloth is fasten on the rubbing roller with the best direction of inclination of the pile.